Conventional magnetoresistive (MR) sensors, such as those used in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin of the GMR effect is that the application of an external magnetic field causes a variation in the relative orientation of the magnetic moments in neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetic moments of the ferromagnetic layers changes.
GMR in conventional artificial multilayer structures that are formed of alternating layers of ferromagnetic and nonferromagnetic metals can be described by two basic geometries. In the first, as depicted in FIG. 1a, the multilayer 1 is formed vertically on a substrate 2 and the applied electrical current is supplied in the plane of the multilayer 1 from electrical lead 3 to electrical lead 4. The leads 3, 4 are defined by conventional lithography on the top of the multilayer 1. In the second, as depicted in FIG. 1b, the the multilayer 5 is formed formed vertically on a conductive layer 6 on substrate 7 and the electrical current is supplied between leads 8 and 9 to flow perpendicular to the interfaces between the ferromagnetic and nonferromagnetic layers of the multilayer. The structure of FIG. 1b requires lithographic isolation of small-dimension vertical columns in the multilayer stack so that current is carried predominantly by electrons which retain spin polarization between the neighboring ferromagnetic layers. There are several problems with both of the structures of FIGS. 1a and 1b. Because the individual layers must be formed sequentially one on top of the other to form the vertical stack of layers it is difficult to obtain the relatively precise thickness required to assure antiferromagnetic coupling between adjacent ferromagnetic layers across the intermediate nonferromagnetic layer. It is also difficult to make electrical connection from the wiring patterns that are formed on the substrate to the lead or leads on the top of an isolated multilayer stack. In addition, the total magnetic thickness of the multilayer, which is directly related to the sensitivity of the sensor, needs to be limited to support increased linear density of recorded data. Consequently there is a severe limit to the number of bilayers in the multilayers depicted in FIGS. 1a and 1b.
What is needed is a GMR multilayer structure that is relatively easy to fabricate and does not have the number of its layers limited by the need to achieve a high linear data density.